TARDIGRADES OF THE GULF OF MEXICO 



By B. G. Chitwood, formerly. Department of Biology, The Catholic University of America 



Tardigrades are small, segmented organisms 

 (usually under 1 mm. in length) showing some 

 similarities to arthropods and annelids. They 

 have four pairs of parapodia and a varied number 

 of sensory appendages. The body is covered 

 with a cuticle of uncertain composition ; this layer 

 is molted during development. The musculature 

 is apparently smooth, though it presents many 

 similarities to striated muscle. The mesoderm 

 arises as four pairs of gut pockets, and the body 

 cavity is a haemocoele containing a colorless 

 liquid with numerous cells in suspension. A 

 circulatory system is absent. Respiration is 

 through the cuticle. These organisms are very 

 sensitive to lack of oxygen. The nervous system 

 consists of a large, lobed dorsal and lateral gang- 

 lion, paired commissures, and a paired ventral 

 nerve with four segmentally paired ganglia. 



The digestive tract consists of an ectodermal 

 buccal tube, bulb and esophagus, a mesenteron, 

 and ventral anus. Opening on each side into the 

 buccal tube there is a protrusible stylet. Salivary 

 glands also open into the buccal cavity. The bulb 

 acts as a pump. These forms usually feed on the 

 contents of chlorophyll cells punctured by the 

 stylets ; digestion in the mesenteron is intracellular. 

 Posteriorly, there are a group of unicellular to 

 multicellular rectal glands or malpighian tubules at 

 the junction of mesenteron and rectum in the 

 fresh-water forms (Eutardigrada), while such are 

 absent in the marine forms (Heterotardigrada). 

 This is an interesting feature relative to osmo- 

 regulation and excretion. We have similar lack 

 of development of rectal glands and lateral 

 excretory canals in the marine nematodes; the 

 protonephridial system of marine Turbellaria is 

 less developed than in fresh-water forms; and the 

 contractile vacuoles of Protozoa are only developed 

 in fresh-water forms. 



Sexes are separate, there being a single tuboid 

 gonad suspended dorsal to the intestine. In the 

 male two deferent tubes encircle the intestine to 



open in a single ventral gonopore anterior to the 

 anus. In the female the single oviduct opens 

 anterior to the anus in the Heterotardigrada and 

 into the rectum in the Eutardigrada. The eggs 

 are usually ornate. Cleavage is total, equal with 

 the formation of a blastula. 



Tardigrades usually inhabit algae and mosses. 

 Fresh-water forms are known to be able to resist 

 extensive drying and extremely low temperatures 

 in a state of anabiosis. The two most compre- 

 hensive reference articles on the group are those 

 by Marcus (1929) and Cuenot (1932). 



The group has been placed with the Acarina, 

 Onychophora, Annelida, and Nematoda. Most of 

 the characters are similar to those of arthropods, 

 but the musculature, digestive tract, and absence 

 of circulatory system present difficulties. For the 

 present, it appears best to place these organisms 

 as a phylum under the Subkingdom Annulosa but 

 to recognize similarities to the Subkingdom 

 Scolecida. 



A single species Bathyechiniscus tetronyx Steiner, 

 1926, has been reported from sargassum in the 

 vicinity of Aransas Bay, Texas (Chitwood 1951). 

 This species has also been reported from the South 

 Polar regions (Steiner 1926) and in Dictyota wash- 

 ings on the California coast. 



BIBLIOGRAPHY 



Chitwood, B. G. 



1951. A marine tardigrade from the Gulf of Me.xico. 

 Texas Jour. Sci. 3 (1): 111, 1 fig. 

 Cuenot, L. 



1932. Tardigrades. Faune de France 24: 1-96'. 

 Marcus, E. 



1929. Tardigrada. Bronn's Klassen u. Ordnung des 

 Tierreichs 5 (4) : 1-608. 

 Mathews, G. C. 



1938. Tardigrada from North America. Am. Midi. 

 Nat. 19: 619-627. 

 Steiner, G. 



1926. Bathyechiniscus tetronyx n. g., n. sp. Ein neuer 

 mariner Tardigrade. Deutsche Sudpolar — Exped. 

 1901-1903, 18: Zool. (10): 479-481. 



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